1. Field of the Invention
The present invention relates to medical monitoring systems and, in particular, to a highly portable, non-invasive, simple to use, physiological sensor and sensor support apparatus. In addition to critical care monitoring, this apparatus can be utilized to comfortably and reliably monitor patients who are ambulatory or sleeping.
2. Discussion of the Prior Art
In the medical monitoring area, single and multi-channel electrocardiographic patient monitoring apparatus are currently available such as those manufactured by Hewlett-Packard Company of Palo Alto, Calif.; SpaceLabs of Redmond, Wash.; and Marquette Electronics of Milwaukee, Wis.
These types of systems require the patient's skin to be first vigorously scrubbed in preparation for gluing probes on or applying probes to the patient's skin with a "wet" process, or the systems use bulky apparatus to hold the probes in place. If these probes lose electrical contact with the patient's skin, trained personnel are needed to re-prepare the skin and re-attach the probes.
"Holter monitors" involve the use of a tape recorder to record signals from a number of "wet" electrode sensors which are hard-wired to the tape recorder. Uncomfortable sensors, wires, and equipment can dig into the patient's skin and restrict mobility.
Non-Invasive Monitoring Systems, Inc. (NIMS) of Miami Beach, Fla., manufactures a cardiorespiratory monitoring system called RESPITRACE PLUS which uses wet electrodes to monitor ECG and a pair of bands to measure respiration through inductance plethysmography. Inductance plethysmography, as implemented in the NIMS system, involves the use two bands. In each band, a wire is laid down in a sinusoidal pattern and sewn into a stretch fabric. An oscillator, having fragile connectors that tend to break readily, supplies a signal which is applied to the wire. As the signal propagates through the wire, changes in the shape of the wire caused by patient respiration, or other movement, cause changes in the signal out of the wire. Recent modifications to the NIMS product include the addition of one channel to measure ECG using traditionally attached electrodes. The bands used to measure respiration are not very durable and laundering the bands to sterilize them is difficult. Further, the wire-carrying band and oscillator create a bumpy surface which can cause discomfort to a patient when lying on them.
The monitoring apparatus manufactured by American Health Products of Orange, Calif. (see U.S. Pat. No. 4,889,131) and Polar U.S.A. of Stamford, Conn. eliminate the wet electrode because both include a belt worn around the torso of a patient, and hold sensors to monitor respiration and/or ECG signals. In the products of American Health Products, a sensor pair is used to measure multiple parameters. In the Polar products, a sensor pair is used to measure ECG only. Both belts have an elastic portion, but the portions of the belts that carry the sensors and electronics are "non-stretchable."
Polar has recently introduced a "Heart Bra" for use with its Heart Rate Monitors. The product is made of fabric and includes an elastic band to which two electrodes are mounted. It is believed that the elastic band has a loop which holds the electrode in a position between the elastic band and the user's skin, so that most of the electrode is in contact with the user's skin.
The American Health Products device uses impedance pneumography and houses the sensors in hard plastic modules that can be uncomfortable because they cannot conform to the patients body. None of the systems noted above are fully compliant or elastic and therefore can cause significant discomfort when worn.
Monitors which rely upon single band "impedance pneumography", such as the apnea monitor products manufactured by Healthdyne of Marietta, Ga., use artifact-prone electrodes. A single band system does not detect "obstructive apnea", a condition where the airway is obstructed and air is merely exchanged between the abdominal and thoracic cavities rather than replenished from the outside. Because these respiration sensors consist only of a chest band, the abdominal component of breathing is neglected.
With most of these monitoring systems, the major cause of false data, and thus false alarms, is artifact (signal noise). Artifact is typically generated by: patient activity; lead movement; rubbing of skin and clothing over leads or electrodes; or, in monitoring systems using radio telemetry, by electromagnetic interference (EMI). In the "stick on" or "wet" electrode systems the movement is caused by the electrode lead wires being pulled on thus causing the electrode to be separated from the patient's skin. It is frequently not obvious from a visual inspection of an electrode that it has come loose and an impedance check must then be performed to determine which electrode is loose. It is believed that in the band supported systems the movement of sensors relative to the patient's skin is contributed to by the non-elasticity of the support belt.
Therefore, with existing systems, the patient's mobility is typically intentionally limited to reduce artifact. Further, the physical constraints of the apparatus also operate to limit the patient's mobility. It is also believed that none of the prior art allows practical monitoring of an ambulatory patient over an extended period of time.